It has been a while since my last contribution. The following patch allows
GCC's optimizers
to more aggressively eliminate and optimize java array bounds checks. The
results are
quite impressive, for example producing a 26% performance improvement on the
sieve.java
benchmark given at http://keithlea.com/javabench/ on x86_64-pc-linux-gnu,
reducing the
runtime for 1 million iterations from 31.5 seconds on trunk, to 25.0s with this
patch, in fact
eliminating all array bounds checks. This is close to the 22.3s of an
equivalent C/C++
implementation, and significantly closes the gap to Java HotSpot(TM) JIT at
23.0 seconds.
The approach is to provide sufficient information in the gimple generated by
the gcj front-end
to allow the optimizers to do their thing. For array allocations of constant
length, I propose
generating an additional (cheap) write to the array length field returned from
_Jv_NewPrimArray,
which is then sufficient to allow this value to propagate throughout the
optimizers.
This is probably best explained by a simple example. Consider the array
initializer below:
private static int mk1[] = { 71,85,95 };
which gets compiled into the java byte code sequence below:
0: iconst_3
1: newarray int
3: dup
4: iconst_0
5: bipush 71
7: iastore
8: dup
9: iconst_1
10: bipush 85
12: iastore
13: dup
14: iconst_2
15: bipush 95
17: iastore
18: putstatic #3 // Field mk1:[I
21: return
Currently, the .004t.gimple generated by gcj for the array allocation is the
cryptic:
#slot#0#0 = 3;
#ref#0#2 = _Jv_NewPrimArray (&_Jv_intClass, #slot#0#0);
#ref#1#4 = #ref#0#2;
_ref_1_4.6 = #ref#1#4;
which unfortunately doesn't provide many clues for the middle-end, so we end up
generating the following .210t.optimized:
void * _3 = _Jv_NewPrimArray (&_Jv_intClass, 3);
int _4 = MEM[(struct int[] *)_3].length;
unsigned int _5 = (unsigned int) _4;
if (_4 == 0)
goto <bb 3>;
else
goto <bb 4>;
<bb 3>:
_Jv_ThrowBadArrayIndex (0);
<bb 4>:
MEM[(int *)_3 + 12B] = 71;
if (_5 == 1)
goto <bb 5>;
else
goto <bb 6>;
<bb 5>:
_Jv_ThrowBadArrayIndex (1);
<bb 6>:
MEM[(int *)_3 + 16B] = 85;
if (_5 == 2)
goto <bb 7>;
else
goto <bb 8>;
<bb 7>:
_Jv_ThrowBadArrayIndex (2);
<bb 8>:
MEM[(int *)_3 + 20B] = 95;
mk1 = _3;
return;
which obviously contains three run-time array bounds checks. These same checks
appear
in the x86_64 assembly language:
subq $8, %rsp
xorl %eax, %eax
movl $3, %esi
movl $_Jv_intClass, %edi
call _Jv_NewPrimArray
movl 8(%rax), %edx
testl %edx, %edx
je .L13
cmpl $1, %edx
movl $71, 12(%rax)
je .L14
cmpl $2, %edx
movl $85, 16(%rax)
je .L15
movl $95, 20(%rax)
movq %rax, _ZN9TestArray3mk1E(%rip)
addq $8, %rsp
ret
.L13: xorl %edi, %edi
xorl %eax, %eax
call _Jv_ThrowBadArrayIndex
.L15: movl $2, %edi
xorl %eax, %eax
call _Jv_ThrowBadArrayIndex
.L14: movl $1, %edi
xorl %eax, %eax
call _Jv_ThrowBadArrayIndex
With the patch below, we now generate the much more informative .004t.gimple
for this:
D.926 = _Jv_NewPrimArray (&_Jv_intClass, 3);
D.926->length = 3;
This additional write to the array length field of the newly allocated array
enables much
more simplification. The resulting .210t.optimized for our array
initialization now becomes:
struct int[3] * _3;
_3 = _Jv_NewPrimArray (&_Jv_intClass, 3);
MEM[(int *)_3 + 8B] = { 3, 71, 85, 95 };
mk1 = _3;
return;
And the x86_64 assembly code is also much prettier:
subq $8, %rsp
movl $3, %esi
movl $_Jv_intClass, %edi
xorl %eax, %eax
call _Jv_NewPrimArray
movdqa .LC0(%rip), %xmm0
movq %rax, _ZN9TestArray3mk1E(%rip)
movups %xmm0, 8(%rax)
addq $8, %rsp
ret
.LC0: .long 3
.long 71
.long 85
.long 95
Achieving this result required two minor tweaks. The first is to allow the
array length constant
to reach the newarray call, by allowing constants to remain on the quickstack.
This allows the
call to _Jv_NewPrimArray to have a constant integer argument instead of the
opaque #slot#0#0.
Then in the code that constructs the call to _Jv_NewPrimArray we wrap it in a
COMPOUND_EXPR
allowing us to insert the superfluous, but helpful, write to the length field.
Whilst working on this improvement I also noticed that the array bounds checks
we were
initially generating could also be improved. Currently, an array bound check
in 004t.gimple
looks like:
D.925 = MEM[(struct int[] *)_ref_1_4.6].length;
D.926 = (unsigned int) D.925;
if (_slot_2_5.9 >= D.926) goto <D.927>; else goto <D.921>;
<D.927>:
_Jv_ThrowBadArrayIndex (_slot_2_5.8);
if (0 != 0) goto <D.928>; else goto <D.921>;
<D.928>:
iftmp.7 = 1;
goto <D.922>;
<D.921>:
iftmp.7 = 0;
<D.922>:
Notice the unnecessary "0 != 0" and the dead assignments to iftmp.7 (which is
unused).
With the patch below, we now not only avoid this conditional but also use
__builtin_expect
to inform the compiler that throwing an BadArrayIndex exception is typically
unlikely. i.e.
D.930 = MEM[(struct int[] *)_ref_1_4.4].length;
D.931 = D.930 <= 1;
D.932 = __builtin_expect (D.931, 0);
if (D.932 != 0) goto <D.933>; else goto <D.934>;
<D.933>:
_Jv_ThrowBadArrayIndex (0);
<D.934>:
The following patch has been tested on x86_64-pc-linux-gnu with a full make
bootstrap
and make check, with no new failures/regressions.
Please let me know what you think (for stage 1 once it reopens)?
Roger
--
Roger Sayle, Ph.D.
CEO and founder
NextMove Software Limited
Registered in England No. 07588305
Registered Office: Innovation Centre (Unit 23), Cambridge Science Park,
Cambridge CB4 0EY
2016-02-21 Roger Sayle <ro...@nextmovesoftware.com>
* expr.c (push_value): Only call flush_quick_stack for non-constant
arguments.
(build_java_throw_out_of_bounds_exception): No longer wrap calls
to _Jv_ThowBadArrayIndex in a COMPOUND_EXPR as no longer needed.
(java_check_reference): Annotate COND_EXPR with __builtin_expect
to indicate that calling _Jv_ThrowNullPointerException is unlikely.
(build_java_arrayaccess): Construct an unlikely COND_EXPR instead
of a TRUTH_ANDIF_EXPR in a COMPOUND_EXPR. Only generate array
index MULT_EXPR when size_exp is not unity.
(build_array_length_annotation): When optimizing, generate a write
to the allocated array's length field to expose constant lengths
to GCC's optimizers.
(build_newarray): Call new build_array_length_annotation.
(build_anewarray): Likewise.
Index: expr.c
===================================================================
--- expr.c (revision 233190)
+++ expr.c (working copy)
@@ -37,6 +37,7 @@
#include "jcf.h"
#include "parse.h"
#include "tree-iterator.h"
+#include "tree-eh.h"
static void flush_quick_stack (void);
static void push_value (tree);
@@ -54,6 +55,7 @@
static void expand_java_pushc (int, tree);
static void expand_java_return (tree);
static void expand_load_internal (int, tree, int);
+static void expand_store_internal (tree, int, int);
static void expand_java_NEW (tree);
static void expand_java_INSTANCEOF (tree);
static void expand_java_CHECKCAST (tree);
@@ -273,10 +275,12 @@
/* If the value has a side effect, then we need to evaluate it
whether or not the result is used. If the value ends up on the
quick stack and is then popped, this won't happen -- so we flush
- the quick stack. It is safest to simply always flush, though,
- since TREE_SIDE_EFFECTS doesn't capture COMPONENT_REF, and for
- the latter we may need to strip conversions. */
- flush_quick_stack ();
+ the quick stack. It is safest to always flush non-constant
+ operands. */
+ if (! TREE_CONSTANT (value)
+ || TREE_SIDE_EFFECTS (value)
+ || tree_could_trap_p (value))
+ flush_quick_stack ();
}
/* Pop a type from the type stack.
@@ -778,19 +782,13 @@
{
tree node;
- /* We need to build a COMPOUND_EXPR because _Jv_ThrowBadArrayIndex()
- has void return type. We cannot just set the type of the CALL_EXPR below
- to int_type_node because we would lose it during gimplification. */
+ /* _Jv_ThrowBadArrayIndex() has void return type. */
gcc_assert (VOID_TYPE_P (TREE_TYPE (TREE_TYPE (soft_badarrayindex_node))));
node = build_call_nary (void_type_node,
- build_address_of (soft_badarrayindex_node),
- 1, index);
+ build_address_of (soft_badarrayindex_node),
+ 1, index);
TREE_SIDE_EFFECTS (node) = 1;
-
- node = build2 (COMPOUND_EXPR, int_type_node, node, integer_zero_node);
- TREE_SIDE_EFFECTS (node) = 1; /* Allows expansion within ANDIF */
-
- return (node);
+ return node;
}
/* Return the length of an array. Doesn't perform any checking on the nature
@@ -833,10 +831,12 @@
{
if (!flag_syntax_only && check)
{
+ tree test;
expr = save_expr (expr);
- expr = build3 (COND_EXPR, TREE_TYPE (expr),
- build2 (EQ_EXPR, boolean_type_node,
- expr, null_pointer_node),
+ test = build2 (EQ_EXPR, boolean_type_node, expr, null_pointer_node);
+ test = build_call_expr (builtin_decl_implicit (BUILT_IN_EXPECT), 2,
+ test, boolean_false_node);
+ expr = build3 (COND_EXPR, TREE_TYPE (expr), test,
build_call_nary (void_type_node,
build_address_of (soft_nullpointer_node),
0),
@@ -865,7 +865,7 @@
tree
build_java_arrayaccess (tree array, tree type, tree index)
{
- tree node, throw_expr = NULL_TREE;
+ tree node;
tree data_field;
tree ref;
tree array_type = TREE_TYPE (TREE_TYPE (array));
@@ -882,9 +882,9 @@
{
/* Generate:
* (unsigned jint) INDEX >= (unsigned jint) LEN
- * && throw ArrayIndexOutOfBoundsException.
+ * ? throw ArrayIndexOutOfBoundsException : INDEX.
* Note this is equivalent to and more efficient than:
- * INDEX < 0 || INDEX >= LEN && throw ... */
+ * INDEX < 0 || INDEX >= LEN ? throw ... : INDEX. */
tree test;
tree len = convert (unsigned_int_type_node,
build_java_array_length_access (array));
@@ -893,19 +893,14 @@
len);
if (! integer_zerop (test))
{
- throw_expr
- = build2 (TRUTH_ANDIF_EXPR, int_type_node, test,
- build_java_throw_out_of_bounds_exception (index));
- /* allows expansion within COMPOUND */
- TREE_SIDE_EFFECTS( throw_expr ) = 1;
+ test = build_call_expr (builtin_decl_implicit (BUILT_IN_EXPECT), 2,
+ test, boolean_false_node);
+ index = build3(COND_EXPR, int_type_node, test,
+ build_java_throw_out_of_bounds_exception (index),
+ index);
}
}
- /* If checking bounds, wrap the index expr with a COMPOUND_EXPR in order
- to have the bounds check evaluated first. */
- if (throw_expr != NULL_TREE)
- index = build2 (COMPOUND_EXPR, int_type_node, throw_expr, index);
-
data_field = lookup_field (&array_type, get_identifier ("data"));
ref = build3 (COMPONENT_REF, TREE_TYPE (data_field),
@@ -919,9 +914,11 @@
/* Multiply the index by the size of an element to obtain a byte
offset. Convert the result to a pointer to the element type. */
- index = build2 (MULT_EXPR, sizetype,
- fold_convert (sizetype, index),
- size_exp);
+ index = fold_convert (sizetype, index);
+ if (! integer_onep (size_exp))
+ {
+ index = build2 (MULT_EXPR, sizetype, index, size_exp);
+ }
/* Sum the byte offset and the address of the data field. */
node = fold_build_pointer_plus (node, index);
@@ -1026,6 +1023,34 @@
return indexed_type;
}
+/* When optimizing, wrap calls to array allocation functions taking
+ constant length arguments, in a COMPOUND_EXPR, containing an
+ explict assignment of the .length field, for GCC's optimizers. */
+
+static tree
+build_array_length_annotation (tree call, tree length)
+{
+ if (optimize
+ && TREE_CONSTANT (length)
+ && is_array_type_p (TREE_TYPE (call)))
+ {
+ tree type, note;
+ type = TREE_TYPE (call);
+ call = save_expr(call);
+ note = build3 (COMPONENT_REF, int_type_node,
+ build1 (INDIRECT_REF, TREE_TYPE (type), call),
+ lookup_field (&TREE_TYPE (type),
+ get_identifier ("length")),
+ NULL_TREE);
+ note = build2 (MODIFY_EXPR, int_type_node, note, length);
+ TREE_SIDE_EFFECTS (note) = 1;
+ call = build2 (COMPOUND_EXPR, TREE_TYPE (call), note, call);
+ TREE_SIDE_EFFECTS (call) = 1;
+ }
+ return call;
+}
+
+
/* newarray triggers a call to _Jv_NewPrimArray. This function should be
called with an integer code (the type of array to create), and the length
of the array to create. */
@@ -1033,7 +1058,7 @@
tree
build_newarray (int atype_value, tree length)
{
- tree type_arg;
+ tree type_arg, call;
tree prim_type = decode_newarray_type (atype_value);
tree type
@@ -1045,9 +1070,10 @@
some work. */
type_arg = build_class_ref (prim_type);
- return build_call_nary (promote_type (type),
+ call = build_call_nary (promote_type (type),
build_address_of (soft_newarray_node),
2, type_arg, length);
+ return build_array_length_annotation (call, length);
}
/* Generates anewarray from a given CLASS_TYPE. Gets from the stack the size
@@ -1061,12 +1087,14 @@
tree_fits_shwi_p (length)
? tree_to_shwi (length) : -1);
- return build_call_nary (promote_type (type),
- build_address_of (soft_anewarray_node),
- 3,
- length,
- build_class_ref (class_type),
- null_pointer_node);
+ tree call = build_call_nary (promote_type (type),
+ build_address_of (soft_anewarray_node),
+ 3,
+ length,
+ build_class_ref (class_type),
+ null_pointer_node);
+
+ return build_array_length_annotation (call, length);
}
/* Return a node the evaluates 'new TYPE[LENGTH]'. */
